One of the best methods of being assertive and confident is to know your subject matter. No matter what the situation is, the more you know, the more confident you'll be.
It doesn't matter if it's a job interview or a presentation to your peers, knowing what you're speaking about will give you that aura of confidence you desire.
Be prepared for potential questions on your subject matter. A good idea is to have a trusted coworker help you identify common questions and answers. You will gain confidence in front of others when you first brainstorm and practice with someone you are comfortable with.